The invention concerns a heat exchanger for fruits, vegetables and other agricultural products whereby the product may be heated by a close association with steam. Ordinarily the product is usually triturated, crushed, mashed or otherwise cut to the form of small pieces and juice. To simplify the description we will hereinafter refer to the invention in relationship to its use in heating tomatoes and particularly to tomatoes which are triturated, cut or otherwise crushed.
The heat exchangers for triturated tomatoes presently in use are for the most part of the tube-bundle or tube-chest type. That is, they are equipped with a number of tubes anchored to two tube end plates and enclosed in a generally cylindrical shell. Ordinarily the tubes have a diameter of about 20 to 50 mm. Heating fluid, usually steam and frequently super heated steam, is fed to the cylindrical shell while the triturated tomatoes to be heated are passed through the inside of the tubes in the tube-chest. The heat exchange, of course, is accomplished by the tomatoes and the steam being in close heat exchange relationship on opposite sides of the tube surfaces.
Because of the rapid heat exchange through the walls of the tubes the temperature on the inside surface of the tube is frequently well above 150.degree. C. Such high temperature may cause the product to "burn on" to the inside walls of the tubes, particularly if the product movement along the walls is slow.
In recent decades, the harvesting of agricultural products has been extensively mechanized, even to the extent of developing special varieties of plants whose fruits ripen simultaneously so that the entire plant can be harvested and the fruit stripped from the whole plant. Such mechanization has many advantages but one disadvantage is that the mechanized harvester frequently collects more than the desired products. In addition to the product, mechanical harvesters often collect unwanted vines, weeds, sticks, grass and particularly portions of the very plant from which the product is harvested. Because of these harvesting methods, extensive steps are taken by the processors of agricultural products to cull the harvested products from the debris. Even so, significant quantities of debris frequently follow the harvest into the processing apparatus. It is impossible to avoid the presence of plant parts, weeds, vines, grass, etc. with the product to be processed.
In the case of processing which requires heat exchange apparatus the problem is particularly acute. Much of the debris, such as the twigs, roots or other parts of the harvested plant, has an elongated shape. In the case of the tube nest heat exchanger which has been in use all through the development of mechanical harvesting, the elongated debris has the tendency to block the entrance of the individual tubes thereby slowing the flow of the product therethrough. In addition, if and when the debris itself enters the tube, its dimension is often such that it is squeezed between the opposite walls of the tube thereby creating friction and a slowing of the flow.
As mentioned above, the flow of product through the tubes of the tube nest increases the possibility of product burn-on the inside of the tubes. The increased burn-on itself forms an obstruction within the tubes which, of course, further lowers the flow rate and causes even more burn-on. Moreover, the reduced flow rate increases the pressure drop through the heat exchanger. In order to remove scale, foreign material and burned-on product, the tube-nest heat exchanger must be shut down and dismantled for cleaning. The interior of the individual tubes are then brushed out with appropriate mechanical means. This step is usually very laborious and involves a substantial decrease in the productivity of the system. Because of the increased burn-on and increased pressure drop due to the presence of foreign matter in the tube-chest it is necessary to shut down, dismantle and clean the tubes of the heat exchanger far more frequently than would otherwise be the case.